Short period seismometers with a natural period of about 1 Hz have conventionally been made by mounting a mass on a spring system with suitable damping, and by employing a moving coil and magnet arrangement which delivers output proportional to the rate of displacement of the mass with respect to the mounting frame. This type of seismometer enjoys a high degree of perfection, but suffers from the disadvantage that it is expensive to manufacture. Accordingly, there has been considerable effort in recent years to provide a satisfactory short period seismometer which will be less expensive to manufacture. One approach to this problem has been to use a ceramic transducer disc attached to the spring disc diaphragms which support the mass with respect to the frame. Electrodes applied to the ceramic disc deliver a potential which is proportional to the stress in a ceramic, and therefore, proportional to flexure strain of the diaphragm on which it is mounted.
The prior art includes a number of patents showing this type of suspension. For example, U.S. Pat. No. 3,893,342 to Florian et al shows the mounting of a displaceable accelerometer mass on a pair of crossed beams to which ceramic piezoelectric strips have been bonded. U.S. Pat. No. 3,911,388 to Crump et al, shows an accelerometer comprising a mass in the form of a sphere which is confined within the supporting frame between two metallic discs to which the sphere is tangent, the discs having ceramic material bonded to their outer surfaces. U.S. Pat. No. 3,732,446 to Bryant, shows a mass which is mounted for vibration in a liquid filled chamber, and wherein the liquid transfers vibrations from the mass to a disc 13 which supports a piezoelectric transducer bonded thereto, although the disc is not directly connected with the mass.
The self capacitance of a piezoelectric transducer of the type under present consideration is a problem in that the capacitance goes up as the diameter of the piezoelectric disc is increased. A high impedance electronic amplifier is therefore required to achieve maximum coupling to the piezoelectric disc while providing a low impedance output to drive the cables by which the seismometer is connected to remote recording equipment. Amplifiers of this type are shown in U.S. Pat. No. 3,749,946 to Ruti, and in U.S. Pat. No. Re. 28,596 to Siegel.